1. Field of the Invention
The present invention relates to a refrigerating machine oil composition, particularly relates to a refrigerating machine oil composition characterized in containing a hydrocarbon oil and an alicyclic polycarboxylic acid ester compound.
2. Related Background Art
Owing to the restriction of fluorocarbons for preventing ozone layer destruction and prevention of global warming, efforts are being made to replace chlorine-containing refrigerants such as CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon) with HFC (hydrofluorocarbon), and to realize high efficiency of a refrigerating system. On the other hand, since HFC refrigerants are also the objects under the restriction for preventing global warming, the application of natural refrigerants such as carbon dioxide, ammonia, hydrocarbons have been researched.
In accordance with the efforts to substitute the above-mentioned refrigerants for the conventional refrigerants, various refrigerating machine oils have been developed. These refrigerating machine oils must satisfy a number of performance requirements including lubricity, refrigerant miscibility, heat stability/hydrolysis resistance, electric insulating ability, low hygroscopicity and the like. Therefore, the compounds contained in the refrigerating machine oils are selected to meet the required performances on the basis of the type and the use of the refrigerants. For example, refrigerating machine oils for HFC refrigerants may contain oxygen compounds such as esters, ethers and carbonates that are miscible with the refrigerant, or they may contain alkylbenzene having inferior miscibility with the refrigerant but having excellent lubricity, heat stability and hydrolysis resistance.
In order to achieve a high efficiency of a refrigerating system, the efforts have been made to lower the viscosity of the refrigerating machine oil. The ester refrigerating machine oils, such as polyol ester obtained from the reaction between an aliphatic polyhydric alcohol and a fatty acid, are disclosed in Japanese Translation Publication No. Hei 3-505602 (JP-A 3-505602) of International Publication for Patent Application and Japanese Patent Kokai (Laid-Open) Publication No. Hei 3-128991 (JP-A 3-128991). One effective means for lowering viscosity of such kinds of ester refrigerating machine oils is to select a fatty acid having a small number of carbon atoms in its alkyl group. However, in general, an ester obtained from a fatty acid with a small alkyl group would have low heat stability and hydrolysis resistance.
Japanese Patent Kokai (Laid-Open) Publication No. Hei 9-221690 (JP-A 9-221690) discloses an alicyclic polycarboxylic acid ester which is an ester refrigerating machine oil having excellent heat stability and hydrolysis resistance. However, even the ester having this structure has not sufficient heat stability and hydrolysis resistance when the viscosity thereof is lowered.
Therefore, an ester refrigerating machine oil that has both low viscosity for obtaining a high efficiency and high heat stability/hydrolysis resistance, and can also satisfy other required performances has not be developed yet.